One important feature in relational database system (RDBMS) is the ability to perform queries that join columns from two or more tables. An example of a query including a join is:
SELECT T1.*, T2.* FROM T1, T2 WHERE T1.A equality_condition T2.B;
where T1 and T2 are tables, T1.A is a column in T1, T2.B is a column in T2, and equality_condition is any condition requiring equality between the operands to the condition. The example query above will return all of the columns in T1 horizontally concatenated with all of the columns in T2, for rows where T1.A and T2.B satisfy the equality condition. In mathematical terms, this query may be described as a Cartesian product with a condition or cross product with a condition.